This invention relates to electron beam welding and, more particularly, to a modified electron beam welding process for joining superalloy materials.
Nickel-base or Cobalt-base superalloys are alloys containing about 50% or more by weight of nickel, plus alloying elements that are added to improve the mechanical and physical properties of these alloys. These materials are typically used in aircraft and industry gas turbine components and other applications that require good strength, creep resistance, fracture toughness, and other mechanical properties at elevated temperatures for extended periods of time.
Single crystal nickel-base superalloys, like Rene N5, that contain greater than 10% refractory elements are generally viewed as unweldable. However, use of a low heat input welding process such as laser or electron beam has produced crack free weld joints over a very narrow range of welding conditions. One drawback to these beam processes is the directional grain growth in the fusion zone that forms a distinct dendritic boundary in the center of the weld zone. This type of grain structure makes the joint vulnerable to centerline cracking and results in poor fatigue strength. For example, the fatigue life of electron beam welded N5 material at 1200xc2x0 F. and 0.9% strain fails at about 100 cycles, which is much lower than that of the base metal. Weld property levels in this range can result in catastrophic failure of the weld joint during operation in a gas turbine.
Alternative processes have been developed for welding the Rene N5 material to overcome the centerline cracking problems. Among them, a wire feed electron beam welding process and a gas tungsten arc welding process (TIG) were the best performers in improving fatigue life of the joint. The wire feed electron beam process adds a ductile filler metal through an automatic wire feeder during electron beam welding. Because of the increased ductility of the weld metal, the fatigue life of the wire feed EB joint was greatly improved. It was evident, however, that this process was limited by the joint thickness. Lack of penetration defects often occurred when the joint thickness was increased to 0.3 inches. The TIG welding process also used a ductile filler metal. This multi-pass arc process completely changed the directional grain structure in the weld zone and also introduced ductile filler metal into the weld metal. This high heat input arc welding process, however, caused relatively large airfoil distortions and increased the risk of lack of fusion defects in the weld. Often, the amount of distortion prohibited the use of the TIG process as the primary welding process for complex airfoil structures.
In an exemplary embodiment of the invention, a method of electron beam welding a joint between superalloy materials includes the steps of inserting a weldable shim in the joint and heating the superalloy materials with an electron beam. The shim may be formed of a superalloy material. For a 0.3 inches thick joint, the heating step preferably comprises powering the electron beam with a voltage between 100-130 kV and a current between 25-35 mA at a speed of 20-30 ipm. With this method, the heating step can be completed in a single pass.
In another exemplary embodiment of the invention, a method of welding a joint gap up to at least 0.040 inches between superalloy materials includes the steps of inserting a shim in the gap and heating the superalloy materials with an electron beam. The joint gap in fact may be up to 0.100 inches.